Solar cells are generally made of semiconductor materials, such as silicon (Si), which convert sunlight into useful electrical energy. Solar cells are typically made of thin wafers of Si in which the required PN junction is formed by diffusing phosphorus (P) or Boron (B) from a suitable phosphorus or Boron source into a P-type or N-type Si wafer. The side of silicon wafer on which sunlight is incident is in general coated with an anti-reflective coating (ARC) to prevent reflective loss of incoming sunlight, and also it acts as passivation layer to prevent the recombination of photo-generated carriers thus to increase the efficiency of the solar cell. A two dimensional electrode grid pattern known as a front contact makes a connection to the N-side or P-side of P-type Si or N-type Si, and a coating of aluminum (Al) or Silver (Ag) on the other side (back contact) makes connection to the P-side or N-Side of the silicon. These contacts are the electrical outlets from the PN junction to the outside load.
Front contacts of silicon solar cells are formed by screen-printing a thick film paste. Typically, the paste contains appropriately fine silver particles, glass and organics. After screen-printing, the wafer and paste are fired in air, typically at furnace set temperatures of 650-1000° C. During the firing, glass softens, melts, and reacts with the anti-reflective coating, etches the silicon surface, and facilitates the formation of intimate silicon-silver contact. Silver deposits on silicon as islands. The shape, size, and number of silicon-silver islands determine the efficiency of electron transfer from silicon to the outside circuit.